Microelectronic devices, such as those fabricated on substrates of single-crystal silicon, typically contain one or more intricately patterned levels of conductors which interconnect the miniature circuitry built into a semiconductor device. These levels of conductors are commonly separated by, and covered with, a film of insulating material. Where the insulating material is inserted between two conducting levels, it is known in the art as an interlevel dielectric (ILD). Where the insulating material separates conductors on the same level, it may be called an intermetal dielectric, or it may be considered with the ILD, particularly if formed by the same deposition as the ILD. Where the insulating material is deposited over the topmost level of conductors on a device, it is known in the art as a protective overcoat (PO). In general, both ILD and PO may be classed as dielectric thin films.
Such dielectric thin films may serve many purposes, including: preventing unwanted shorting of neighboring conductors or conducting levels, by acting as a rigid, insulating spacer; preventing corrosion or oxidation of metal conductors, by acting as a barrier to moisture and mobile ions; filling deep, narrow gaps between closely spaced conductors; and planarizing uneven circuit topography so that a level of conductors can then be reliably deposited on a film surface which is relatively flat. A significant limitation is that typically ILD and PO films must be formed at relatively low temperatures to avoid destruction of underlying conductors. Another very important consideration is that such dielectric films should have a low relative dielectric constant k, as compared to silicon dioxide (k=3.9), to lower power consumption, crosstalk, and signal delay for closely spaced conductors.
Films deposited from hydrogen silsesquioxane (HSQ) resins have been found to possess many of the properties desirable for ILD and PO applications. Haluska et at. (U.S. Pat. No. 4,756,977, Jul. 12, 1988) describe a film deposition technique comprising diluting in a solvent a hydrogen silsesquioxane resin, applying this as a coating to a substrate, evaporating the solvent and ceramifying the coating by heating the substrate in air. Others have found that by ceramifying such a coating in the presence of hydrogen gas (Ballance et at., U.S. Pat. No. 5,320,868, Jun. 14, 1994, which is included herein by reference) or inert gas (European Patent Application 90311008.8), the dielectric constant of the final film may be lowered and/or stabilized as compared to ceramifying in air. Generally, it has been taught that curing in air produces a predominantly Si-O film, curing in ammonia produces a silicon oxynitride type film, and curing in inert or reducing atmospheres results in films which retain some portion of the Si-H bonding inherent in uncured HSQ.